1. Field of the Invention
The invention relates to plastic pistons which are commonly employed in disc brakes and, in particular, plastic pistons made with a metallic cap which protects the open end of the piston that comes into contact with the back face of the friction pad in a disc brake assembly. More specifically, the invention, has to do with an improvement in the metallic cap.
2. The Related Art
Plastic pistons have been employed in vehicular disc brake assemblies for several years. In some applications, a heat resistant ring or metallic cap is employed to protect the plastic from the heat generated during braking.
U.S. Pat. No. 4,449,447 describes a resinous disc brake piston having a heat resistant ring bonded thereto at the opening end thereof. In some embodiments, the rings are perforated for degassing in the process of molding the ring and the piston body.
Metallic caps which are attached by various anchoring means to the open end of the piston are described in Canadian Patents 1,187,424 and 1,198,379.
Canadian Patent 1,228,963 describes phenolic pistons having metallic caps integrally bonded thereon. These pistons are manufactured by placing a cap in a mold and then adding phenolic resinous material which is melted and compressed against the surface of the metallic cap so as to form a firm bond therewith. According to the patent, the bond between the resin body of the piston and the metallic cap can be improved by crimping the edges of the cap so that the edges are impregnated in the resin body. Other patents describing plastic pistons with integrally bonded caps having crimped edges are U.S. Pat. Nos. 5,575,358 and 5,713,435.
The present invention is directed to a new cap design which provides an improved bond between the resin body of the piston and the metallic cap without employing crimped edges on the side walls of the cap as described in the prior art. The invention also prevents or minimizes the undesirable flow of phenolic resin up the outside of the crimped side walls of the cap toward the open end of the piston. Furthermore, handling of the caps during manufacturing is facilitated by the invention because the side walls of the caps have circumferences which are not interrupted by crimped, tabbed or pronged edges. Accordingly, they can be picked up by robotic equipment from any position without being dropped prematurely and they are placed in the mold consistently in the correct position. Other advantages will be apparent to those skilled in the art.
A metallic cap for a plastic disc brake piston is provided with openings in a cylindrical side wall thereof. The plastic is caused to flow into the openings during molding of the piston and thereby create an extension of the plastic material which locks the cap onto the piston. The cap is comprised of an end wall and at least one side wall. In a preferred embodiment the cap has an end wall and inner and outer side walls. The openings are generally in the form of holes or slots and they are provided in either or both of the side walls. At least one opening is required and, preferably, at least two openings are employed opposite one another on the same side wall. The cap is integrally molded to the piston by placing the cap in the mold, adding resin material and causing molding of the resin material by heating and/or compression. During molding some of the resin flows into the openings making extensions of the resin material. This creates an interlock which firmly secures the cap to the piston.